Chalcogen bonding in the solid-state structures of 1,3-bis(benzimidazoliumyl)benzene-based chalcogen-bonding donors

1,3-Bis(benzimidazoliumyl)benzene-based chalcogen-bonding catalysts were crystallized and their intermolecular interactions investigated. Depending on the chalcogen employed and the crystallization conditions, different binding motifs were found. With sulfur centres as Lewis acidic interaction sites, only weak chalcogen bonding was observed. For selenium-based compounds, however, different bidentate coordination motifs were found between the chalcogen-bonding donor and the respective counter-ions.


Introduction
Chalcogen bonding (ChB) (Aakeroy et al., 2019) is defined as the attractive interaction of Lewis acidic chalcogen centres with Lewis bases and is closely related to hydrogen bonding (Doyle & Jacobsen, 2007) and halogen bonding (Cavallo et al., 2016). However, chalcogen bonding is the least investigated interaction in this group. This interaction can be explained by three major contributions, again similar to hydrogen and halogen bonding: (i) charge transfer, described by an n!�*orbital interaction (Mulliken, 1952); (ii) electrostatic attraction of the lone pair of the Lewis base with an electropositive region (�-hole) at the chalcogen (Rosenfield et al., 1977); (iii) dispersion (Bleiholder et al., 2006). In general, chalcogen bonding is a more directional interaction compared to hydrogen bonding, which is explained by the absence of filled p-orbitals at hydrogen and by the fact that the �*-orbital of the R-H bond is comprised of the 1s orbital on the hydrogen side. Furthermore, the interaction site is not limited to a single atom and can vary between tellurium, selenium and sulfur, which consequently allows the fine tuning of the interaction strength. Less electronegativity and easier polarization of the chalcogen leads to an increased anisotropic electron distribution and thus an enlarged electropositive region (�-hole), as well as to a lower-lying �*-orbital of the R-Ch bond. Hence, the strength of the chalcogen bonding increases with the atomic number of chalcogen (S < Se < Te). Another difference to hydrogen and halogen bonding is the second substituent located at the interacting atom. This interaction was observed in several solid-state systems, such as Ebselen, a synthetic organoselenium drug (Dupont et al., 1990) and diphenyl diselenide with iodine (Kubiniok et al., 1988). Intensive studies (Bleiholder et al., 2006) and applications of the Gleiter group used chalcogen-chalcogen interactions to synthesize porous materials, such as nanotubes (Werz et al., 2002), that incorporated other organic molecules like solvents (Werz et al., 2004). In parallel, first applications in solution appeared as intramolecular chalcogen bonding was applied for the rigidification of intermediates to induce chirality (Fujita et al., 1994;Wirth, 1995;Tiecco et al., 2002). Moving from intra-to intermolecular interactions in solution, pioneering works in the field of anion recognition via chalcogen bonding have been performed by Gabbaï and co-workers (Zhao & Gabbaï, 2010) using charged chalcogen-bonding donors, and by Taylor and co-workers with neutral chalcogen-bonding donors (Garrett et al., 2015). Extending the field of anion recognition to anion transport, the group of Matile introduced neutral dithienothiophene (DTT) chalcogen-bonding donors (Benz et al., 2016), which were also employed in oligomeric fashion as transmembrane transporters in further studies (Macchione et al., 2018). Furthermore, with the DTT motif, the first application of chalcogen bonding in organocatalysis was presented by the same group (Benz et al., 2017). Catalytic amounts of these compounds successfully activate quinolines for their reductive hydrogenation to 1,2,3,4-tetrahydroquinoline derivatives. In halogen-bonding organocatalysis, cationic donors were often markedly more active than neutral donors (Jungbauer & Huber, 2015). Therefore, our group reported the use of cationic chalcogen-bonding donors, which contained the more Lewis acidic chalcogen selenium on a 1,3-bis(benzimidazoliumyl)benzene scaffold (Wonner et al., 2017a). These catalysts were used in stoichiometric (Wonner et al., 2017a) and catalytic (Wonner et al., 2017b) activations of carbonhalide bonds. We also successfully demonstrated the catalytic activity of the same catalyst and the sulfur congener in the above-mentioned hydrogenation reaction (Wonner et al., 2019b).
The use of cationic selenium-based Lewis acids was further extended by Wang and co-workers in the activation of carbonyl functionalities (Wang et al., 2019). As the development of chalcogen-bonding catalysts moved on, telluriumbased catalysts were introduced and applied in the activation of trans-�-nitrostyrene (Wonner et al., 2019a) and crotonophenone (Wonner et al., 2020) for Michael reactions, as well as in the activation of a carbon-chloride bond (Steinke et al., 2021) and of imines (Steinke et al., 2022). Lately, hypervalent tellurium-based chalcogen-bonding donors were introduced in organocatalysis (Weiss et al., 2021;Zhou & Gabbaï, 2021). Herein, we present four crystal structures of 1,3-bis(benzimidazoliumyl)benzene-based chalcogen-bonding donors with selenium and sulfur as Lewis acidic centres. Monodentate, bidentate and biaxial intermolecular chalcogen-bonding interactions are observed in these solid-state structures, which are further discussed in terms of previously observed catalytic activity. In addition, we performed DFT calculations to analyze chalcogen bonding in the absence of packing effects observed in the crystal structures of the catalyst system at hand.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. All H atoms were refined using the riding model in idealized positions and isotropic radii, with C-H distances of 0.98 Å and U iso (H) = 1.5U eq (C) for methyl H atoms, and C-H = 0.95 Å and U iso (H) = 1.2U eq (C) for other H atoms.

Discussion
As described in the Introduction, selenium-based chalcogenbonding catalysts deriving from a 1,3-bis(benzimidazoliumyl) benzene frame were applied in several reactions. Whereas the first report describes the application in a stoichiometric activation of a carbon-bromine bond (Wonner et al., 2017a), the following article dealt with the catalytic activation of a carbon-chloride bond (Wonner et al., 2017b). As these reactions rely at least partially on the binding of the catalyst to the halide leaving group, they are typically easier to activate or catalyse than reactions involving neutral organic functional groups like carbonyls or imines. Therefore, the next step was the activation of quinolines in a transfer hydrogenation reaction (Wonner et al., 2019b). However, this benchmark reaction showed surprisingly similar catalytic activities between preorganized and non-preorganized catalysts, as well as between sulfur-and selenium-based chalcogen-bonding donors. On theoretical grounds, the preorganized and the selenium-based variant would have been expected to be noticeably more potent. In particular, the comparable performance of the catalyst differing in the chalcogen atom was puzzling and intriguing, as this could mean that better accessible sulfur variants could be used in catalysis without much loss of turn-over frequency.
To further investigate this issue, the non-preorganized sulfur-and selenium-based catalysts were subjected to crystallization studies to investigate the chalcogen-bonding properties. Crystallization of these compounds proved difficult despite various efforts made while varying parameters that affect the crystallization process, such as temperature, concentration and crystallization techniques. The octyl chains, which are responsible for the good solubility of the catalysts in organic solvents, are most likely the reason for this. To avoid this challenge, the respective all-methylated catalysts (3 S and 3 Se ) were synthesized. The preparation of these compounds followed an already reported route ( Fig. 1) (Wonner et al., 2019b), with 1,3-bis(benzimidazolyl)benzene (Ganta & Chand, 2015) and compound 1  being synthesized according to literature procedures. The next step was the formation of the respective sulfur or selenium urea compounds (2 S and 2 Se ) under basic conditions. The final step, methylation of the urea compound, yielded the all-methylated compounds 3 S and 3 Se .
Next, crystallization studies were carried out. The vapour diffusion method was applied successfully to crystallize compound 3 S from dichloromethane and cyclohexane (Fig. 2). This compound crystallizes in the monoclinic space group P2 1 /c. The unit-cell constants are a = 7.7776 (2), b = 22.5823 (6) and c = 18.2338 (5) Å , with a volume of 3160.16 (15) Å 3 and a density of 1.536 Mg m À 3 . The unit cell is built up of four ion pairs. Within this structure, an intermolecular interaction between atoms S1 and F1 of a trifluoromethanesulfonate anion is observed. The S1� � �F1 distance is 3.154 (3) Å , 96% of the van der Waals radii (�r ! ), and the directionality is rather poor (C1-S1� � �F1 = 146.9 � ). The supposedly more Lewis basic O atoms of the trifluoromethanesulfonate anion are involved in two anion-� interactions with the imidazole unit of benzimidazole. These interactions have distances O6� � � cent = 2.894 (4) Å and O3� � �cent = 3.068 (3) Å from the calculated centroid (cent) of the imidazole unit to the trifluoromethanesulfonate O atoms. From this O atom to the closest C atom, the distances in both cases are 11% shorter than the �r ! (which possibly indicates a stronger interaction compared to the already mentioned chalcogen-bonding interaction). In addition, hydrogen bonding between atoms O4 and H15 in the 2-position of the central arene core [with H15� � �O4 = 2.468 (5) Å , 94% of �r ! , and C15-H15� � �O4 = 166.5 � ] is observed.
Overall, merely a chalcogen-bonding interaction between sulfur and fluorine was observed with a distance that corresponds to 96% of the �r ! . Additional weak hydrogen bonding found in this structure between the backbone H atoms of the arene core and the benzimidazolium unit will not be further discussed in this article.
Under the same crystallization conditions, the seleniumcontaining equivalent 3 Se crystallizes also in the monoclinic space group P2 1 /c (Fig. 3). The unit-cell constants are a = 7.74775 (4), b = 13.12863 (8) and c = 31.27496 (18) Å , with a volume of 3181.02 (3) Å 3 and a density of 1.722 Mg m À 3 . The unit cell contains four ion pairs and bidentate chalcogen bonding from the Se1 and Se2 centres of 3 Se to trifluoromethanesulfonate atom O4 is observed. Some of the anions involved in these interactions are disordered over two positions. The interaction distances are Se1� � �O4 = 3.031 (2) Å and Se2� � �O4 = 3.080 (2) Å , with angles of C3-Se1� � �O4 = 165.31 � and C11-Se2� � �O4 = 159.89 � . The sum of the van der Waals radii is 3.42 Å and therefore the observed distances amount to 89 and 90% of this measure, thus classifying this interaction as stronger chalcogen bonding compared to the interaction found in the structure of 3 S . Atom Se2, which already features chalcogen bonding in the elongation of the benzimidazole C11-Se bond, faces also a second chalcogenbonding interaction to atom O3A of the second trifluoromethanesulfonate anion in the elongation of the CH 3 -Se bond, with an Se2� � �O3A distance of 3.035 (3) Å and a C2-Se2� � �O3A angle of 158.07 � (89% of �r ! ). This additional interaction vividly demonstrates the property of chalcogens to form two noncovalent interactions based on their Lewis acidity. Although only one electron-withdrawing substituent is used, both axes simultaneously seem to be available for chalcogen bonding.
Taking a closer look at the unit cell, several other intermolecular interactions are found. The remaining O atoms (O5 and O6) of this trifluoromethanesulfonate anion are involved in two anion-� interactions to an imidazole group of benzimidazole [O5� � �cent = 3.269 (3) Å and O6� � �cent = 3.307 (3) Å ]. These interactions can be considered very weak, as the distances are close to the actual �r ! with 99 and 100%. Atom O5 faces additional hydrogen bonding, formed with atom H20 sitting in the 4-position of the central arene core [H20� � �O5 = 2.397 (2) Å , 91% of �r ! , and C20-H20� � �O5 = 143.01 � ]. Moreover, atom O1A of the trifluoromethanesulfonate anion is employed in a second hydrogen-bonding interaction to H24, sitting in the 2-position of the central arene core [H24� � �O1A = 2.402 (4) Å , 92% of �r ! , and C24-H24� � �O1A = 161.21 � ]. These two hydrogen-bonding interactions indicate a Lewis acidic character of the benzene H atoms. The overall picture of the unit cell is therefore described by two chalcogen-bonding donors which are bridged with two trifluoromethanesulfonate anions via chalcogen bonding, hydrogen bonding and anion-� interactions. This structure features comparable binding distances within the chalcogen-bonding donor, e.g. the distances of the carbonselenium bonds and also similar distances of the observed intermolecular chalcogen bonding to a related compound (Wonner et al., 2017a). The distances of the anion-� interactions in 3 S are on average 9% shorter than the corresponding interactions found in this crystal structure. This could be explained by the fact that one trifluoromethanesulfonate O atom forms this interaction in the structure of 3 S , whereas two trifluoromethanesulfonate O atoms are involved in the interaction with 3 Se , which geometrically leads to an increased interaction distance. Another explanation could be the more electron-withdrawing properties of sulfur compared to selenium based on the difference in their electronegativity, leading to a more polarized imidazolium system when sulfur is implemented.
Besides this structure, a second crystal structure (B) of 3 Se was obtained from crystallization at lower temperatures (4-5 � C) (Fig. 4). The crystal system is again monoclinic with the space group P2 1 /c. The parameters of the unit cell are a = 13.8634 (4), b = 7.7558 (2) and c = 29.3267 (7) Å , with a volume of 3144.59 (14) Å 3 and a density of 1.742 Mg m À 3 . The unit cell once more consists of four ion pairs. Different to the previously described solid-state structure (A of 3 Se ), each selenium centre features its own chalcogen-bonding interaction to an O atom of a trifluoromethanesulfonate anion. The distances of these interactions are Se1� � �O3 = 3.074 (4) Å (90% of �r ! ) and Se2� � �O1 = 3.052 (4) Å (89% of �r ! ), with C3-Se1� � �O3 = 167.87 � and C5-Se2� � �O1 = 171.26 � . Besides these interactions and differences, yet another short contact is observed. Atom Se1, already featuring chalcogen bonding in the elongation of the benzimidazole C3-Se1 bond, faces also a second chalcogen-bonding interaction to atom O5 of the second trifluoromethanesulfonate anion in the elongation of the CH 3 -Se bond. This interaction [Se1� � �O5 = 3.033 (3) Å , 89% of �r ! , with C3-Se1� � �O5 = 161.19 � ] gives Se1 again a biaxial character. However, only one of the two Se atoms features this biaxial system, as also seen in structure A of 3 Se (Fig. 3). Between the two crystal structures of 3 Se , only minor differences are observed, as the Se� � �O chalcogen-bonding interaction distances are in both cases approximately 90% of the van der Waals radii. The angles of the structure obtained at Structure A of 3 Se with the chalcogen bonding, hydrogen bonding and anion-� interactions marked, and with displacement ellipsoids drawn at the 50% probability level. For the sake of clarity, only H atoms involved in hydrogen bonding are shown. Symmetry operation to generate the second molecule of 3 Se and trifluoromethanesulfonate: À x + 1, À y + 1, À x + 1.

Figure 4
Structure B of 3 Se grown at 4-5 � C, with the chalcogen bonding and hydrogen bonding marked, and with displacement ellipsoids drawn at the 50% probability level. lower temperatures are closer to 170 � . Both structures feature a biaxial chalcogen centre with similar Se� � �O distances (89% of the van der Waals radii) and similar angles below 160 � , which are less directional than the interactions in the elongation of the benzimidazole C-Se bond. Similar to the previously described structures, the 2-position hydrogen (H19) of the central arene core shows its Lewis acidic potential in a hydrogen-bonding interaction to one of the trifluoromethanesulfonate anions (O6) [H19� � �O6 = 2.550 (4) Å , 98% of �r ! , and C19-H19� � �O6 = 157.75 � ].
Changing the solvent in the crystallization experiments from dichloromethane to 1,2-dichloroethane ended up yielding yet another structural variation (structure C of 3 Se ) (Fig. 5). In this case, a triclinic space group P1, with unit-cell parameters of a = 9.3672 (1), b = 13.3265 (2) and c = 13.6445 (1) Å , with a volume of 1647.49 (3) Å 3 and a density of 1.762 Mg m À 3 was obtained. The unit cell is built up of two ion pairs along with four crystallized 1,2-dichloromethane molecules. The binding distances and angles within the donor are comparable to the previously described structure of 3 Se . The crystal structure features two independent intermolecular interactions. The first one includes the Se1 centre and atom Cl1 of 1,2-dichloroethane, with an Se1� � �Cl1 distance of 3.535 (24) Å , which corresponds to 97% of the �r ! , and a C3A-Se1A� � �Cl1A angle of 176 � . The second short contact can be found between the Se2 centre and trifluoromethanesulfonate atom O3, with an Se2� � �O3 distance of 3.133 (4) Å (92% of �r ! ) and a C11-Se2� � �O3 angle of 168 � . This Se� � �O interaction is again comparable to the other observed chalcogen-bonding interactions for 3 Se . The distance of Se1A to the Cl1A atom of 1,2-dichloroethane is very close to the �r ! , which indicates a weaker interaction compared to the interaction of selenium with a trifluoromethanesulfonate O atom. However, since the solvent is not charged and thus a neutral chalcogen-bonding acceptor is involved in the Se� � �Cl interaction, a weaker interaction is expected. The slightly longer distance to the trifluoromethanesulfonate O atom compared to the distance in structures A and B of 3 Se can be explained by the fact that two independent Se� � �O/Cl chalcogenbonding interactions, yet no co-operative effect, are present.
In addition, this structure also features anion-� interactions (Fig. 6). An imidazole moiety of one of the two benzimidazolium systems coordinates with each side to one trifluoromethanesulfonate anion [O1� � �cent = 3.152 (3) Å and O4B� � �cent = 3.063 (4) Å ]. Atom O2 sitting on the same trifluoromethanesulfonate anion as O1 also forms an anion-� interaction [O2� � �cent = 2.920 (3) Å ], becoming therefore a bridging moiety between two chalcogen-bonding donors. The imidazole system of this interaction does not feature a second anion-� interaction. The distances of these interactions are 5-13% shorter than the �r ! of the O atom to the closest C atom. These anion-� interactions are up to 10% shorter compared to those found in structure A of 3 Se (Fig. 4), and provide further indications that the weak anion-� interaction in structure A is due to geometric limitations. Comparing the interaction with 3 S indicates interactions of similar strength, even though selenium is less electronegative and does not polarize the imidazole moiety of benzimidazole in the same fashion as the more electronegative sulfur could do. Structure C of 3 Se , with chalcogen bonding to trifluoromethanesulfonate and 1,2-dichloroethane marked, and with displacement ellipsoids drawn at the 50% probability level. The dichloroethane molecule and trifluoromethanesulfonate anion are generated by (x + 1, y, z).

Figure 6
Structure C of 3 Se , with the anion-� interactions to trifluoromethanesulfonate marked, and with displacement ellipsoids drawn at the 50% probability level. H atoms and 1,2-dichloroethane molecules have been omitted for clarity. The first trifluoromethanesulfonate anion is generated by (x, y, z À 1) and the second trifluoromethanesulfonate anion and 3 Se are generated by (À x + 1, À y + 1, À z).
Considering the fact that the carbon-nitrogen bonds to the central scaffold are free to rotate and therefore the Lewis acidic selenium centres could in principle point in opposite directions, it is interesting to see that a syn-like conformer has been observed in all of the presented structures. The only exception is the disorder found in structure C of 3 Se , with a ratio of 95:5 of the syn-and anti-like conformers. This is especially remarkable for the structure of 3 S and structure C of 3 Se , as no bidentate interaction of the Lewis acidic centres keeps the compound in this conformation, and the last structure of 3 Se , as two independent chalcogen-bonding interactions are found, which in principle could also point in different directions.
Overlaying the structure of 3 S and structure B of 3 Se stresses the similarity of the compounds (Fig. 7). The 1,3-bisbenzimidazolium scaffolds are matching each other in terms of geometry. The biggest difference is that one of the methyl groups located at the chalcogens (right side in this plot) is pointing in opposite direction. Nevertheless, they are roughly occupying the same space. The bigger difference is the orientation of one of the two trifluoromethanesulfonate anions. Whereas for 3 S , the CF 3 group of trifluoromethanesulfonate is facing the sulfur, it is the SO 3 moiety that is facing the selenium centre in structure B of 3 Se .
Besides these solid-state structures, we also performed density functional theory (DFT) calculations to further evaluate and elucidate chalcogen-bonding properties of the herein considered catalyst system. For the in silico investigations, the M06-2X functional (Zhao & Truhlar, 2008) with the triple-zeta def2-TZVP basis set (Weigend & Ahlrichs, 2005) and Grimme's D3 dispersion correction was applied (Grimme et al., 2010;Grimme, 2012). In the electrostatic plots, the energy potentials are set to 87.8 and 153.1 kcal mol À 1 and are projected with 0.001 e Bohr À 3 . The surface map values are summed up in Table 2 in kcal mol À 1 .
First, we consider the 'depth' of the �-holes, which are localized at the elongation of the benzimidazole C-Ch bonds [Figs. 8(a), 8(c) and 8(e)]. According to the expected tendency, the surface map values for these �-holes (at Ch1 and Ch2) are decreasing with the decreasing atomic number of the implemented chalcogen (Table 2). In any case, both �-holes in the described areas show slightly different values (e.g. 152.4 and 153.3 kcal mol À 1 for 3 Te ). The comparison of 3 Se and 3 S is of special interest, as these data can be compared with the previously reported catalysis data (Wonner et al., 2019b). If a deeper �-hole depth is equal to stronger catalytic activity, 3 Se should show an increased activation compared to 3 S . As stated above, this is not the case. Therefore, the observed catalytic activity in the reduction of quinolines cannot be further explained taking only these �-holes into account.
Shining light on the remaining �-holes of these systems, that in the elongation of the CH 3 -Ch bond [Figs. 8(b),8(d) and 8(f)] could provide hints on the origins of the comparable catalytic properties. 3 Te shows defined �-holes [Fig. 8(b)] in this region with decreased absolute values of potentials [with respect to those of the C(benzim)-Ch bond] of 147.8 and 147.2 kcal mol À 1 . Besides this electron-poor area, the H atom in the 2-position of the central arene core also features an electropositive potential of 145.2 kcal mol À 1 .
Moving to 3 Se , these �-holes are less defined and less separated from the electron-poor region at the central arene core [Fig. 8(d)]. In contrast to 3 Te , the four chalcogen-based electron-deficient areas for 3 Se show comparable energies (Table 2, entry 2). The estimated energy for the �-hole of the 2-position H atom shows a potentially more Lewis acidic area compared to those located at the chalcogen. In two of the three crystal structures of 3 Se , we observed weak hydrogen bonding of this H atom. The overall maximum value for 3 Se was observed right between the less defined chalcogen and hydrogen �-hole, with a value of 147.9 kcal mol À 1 . For 3 Te , no such value was found, as they did not overcome the classic �-hole values.
Continuing these investigations with 3 S , no separation of the �-holes (CH 3 -S bond) is spotted and a connected electron- Overlay plot of the structures of 3 S (blue bonds) and 3 Se (structure B) (red bonds).

Table 2
Surface map values for the �-holes of 3 Ch .

Entry
Compound  Table 2, entry 3) are reversed to those from 3 Te , as the 2-position seems to be the most Lewis acidic centre, followed by the CH 3 -S bond �-holes. The C(benzim)-S bond �-holes are those that are the least Lewis acidic. Matching this with the crystal structure of 3 S , these data are confirmed, as no chalcogen bonding in the elongation of the C(benzim)-S bond is found, but one intermolecular action is observed in the elongation of the CH 3 -S bond which is theoretically favoured. Like 3 Se , the most electron-deficient area of 3 S is again observed between the central arene core and the chalcogen. However, in the case of 3 S , the difference to the second most Lewis acidic centre (2.5 kcal mol À 1 ) is larger than the difference in 3 Se (1.0 kcal mol À 1 ). Since sulfur is more electron withdrawing, the larger electron deficient area might be explained by the more electron-withdrawing sulfur, which impacts the whole structure. This might also explain the reason for the catalytic activity in the reduction of quinolones. Nevertheless, it is necessary to mention that various reference experiments were carried out in that reaction which indicate an activation based on chalcogen bonding, for instance, by using the respective hydrogen-bonding equivalents.

Conclusion
In summary, we presented four solid-state structures of sulfurand selenium-containing chalcogen-bonding catalysts. The  structures and their interactions are matched with in silicocalculated respective structures to shine some light on the fairly surprising results of their application in the activation of quinolines. The structure of 3 S features one chalcogen-bonding interaction from sulfur to fluorine of the trifluoromethanesulfonate counter-anion, which has an interaction distance close to the sum of the van der Waals radii, whilst also anion-� interactions and hydrogen bonding are observed. These results are in line with the theoretically predicted data.
In three crystal structures of the selenium-based analogue 3 Se , different mono-and bidentate, as well as biaxial binding motifs to neutral and negatively charged molecules based on chalcogen bonding, are present. Nevertheless, these structures also contained anion-� interactions along with hydrogen bonding. The results could also be related to calculational data. We further extended the DFT calculations to the appropriate tellurium-based compound, which could not be synthesized, and observed the most suitable theoretical properties for chalcogen bonding. However, the unusual activities of the previous report on the reduction of quinolines could not be satisfactorily explained.  (Putz & Brandenburg, 2014) and Mercury (Macrae et al., 2020); software used to prepare material for publication: WinGX (Farrugia, 2012).